Faced with a challenge in the present, you remember an event from your past, realising you were prepared all along.
You must describe through flashback an event in the past, through which you retroactively prepared for a problem facing you in the present. Your description must include how you were able to learn of your current situation and take steps to be ready for it. Your DM will decide the extent to which you were able to prepare, whether any form of skill check is required to measure success, and whether you will need to pay any cost in money, goods, services, reputation etc. to any relevant parties. Your DM has final say on what you can and cannot do with this spell, and may allow you to cancel casting if your intended usage is not possible.
When cast at 1st level you may describe an event no more than 8 hours in the past and acquire no more than 200 gp worth of specialist knowledge, equipment or support as a result of it. For each level above 1st at which this spell is cast you may increase the time by 8 hours and the value by 200 gp.
Examples
- Faced with a password protected magical barrier inside a vault, a player casts Flashback, remembering when they spoke to an informant about the vault's security and learned the password… for a price.
- While preparing to pick a lock after being separated from your party's Rogue, a player uses Flashback to remember that they purchased their own set of Thieves' Tools, just in case.
The idea behind this spell (and the ring and feat that grant it) is to enable Blades in the Dark style retroactive planning for break-ins, heists etc. Instead of wasting a tonne of time on planning, only for everything to go horribly wrong anyway, you can come up with a basic plan for entry then retroactively remember to buy an item, setup an escape route etc.
For general use this spell should be reasonably balanced, and I've limited it to the School of Divination for the time being (though there may be other sub-classes that can justify having it), but it's really intended to be used with the magic ring or feat rather than learned directly.
Reminds me of Honey Heist (google it, a 2 page RPG but I don't think one can post links on D&D Beyond), specifically: